Hybrid qubits enable the hybridization of charge and spin degrees of freedom, which provides a way to realize both a relatively long coherence time and rapid qubit manipulation. Here, we use microwave driving to demonstrate the coherent operation of a tunable hybrid qubit, including X-rotation, Z-rotation, and rotation around an arbitrary axis in the X–Y panel of the Bloch sphere. Moreover, the coherence properties of the qubit and its tunability are studied. The measured coherence time of the X-rotation reaches ∼14.3 ns. While for the Z-rotation, the maximum decoherence time is ∼5.8 ns due to the larger sensitivity to noise. By employing the Hahn echo sequence to mitigate the influence of the low-frequency noise, we have improved the qubit coherence time from ∼5.8 ns to ∼15.0 ns. Our results contribute to a further understanding of the hybrid qubit and a step towards achieving high-fidelity qubit gates in the hybrid qubit.
Hybrid qubits enable the hybridization of charge and spin degrees of freedom, which provides a way to realize both a relatively long coherence time and rapid qubit manipulation. Here, we use microwave driving to demonstrate the coherent operation of a tunable hybrid qubit, including X-rotation, Z-rotation, and rotation around an arbitrary axis in the X–Y panel of the Bloch sphere. Moreover, the coherence properties of the qubit and its tunability are studied. The measured coherence time of the X-rotation reaches ∼14.3 ns. While for the Z-rotation, the maximum decoherence time is ∼5.8 ns due to the larger sensitivity to noise. By employing the Hahn echo sequence to mitigate the influence of the low-frequency noise, we have improved the qubit coherence time from ∼5.8 ns to ∼15.0 ns. Our results contribute to a further understanding of the hybrid qubit and a step towards achieving high-fidelity qubit gates in the hybrid qubit.
Abstract We implement resonant single qubit operations on a semiconductor hybrid qubit hosted in a three-electron Si/SiGe double quantum dot structure. By resonantly modulating the double dot energy detuning and employing electron tunnelling-based readout, we achieve fast (>100 MHz) Rabi oscillations and purely electrical manipulations of the three-electron spin states. We demonstrate universal single qubit gates using a Ramsey pulse sequence as well as microwave phase control, the latter of which shows control of an arbitrary rotation axis on the X–Y plane of the Bloch sphere. Quantum process tomography yields π rotation gate fidelities higher than 93 (96)% around the X (Z) axis of the Bloch sphere. We further show that the implementation of dynamic decoupling sequences on the hybrid qubit enables coherence times longer than 150 ns.
The charge and spin degrees of freedom of an electron constitute natural bases for constructing quantum two level systems, or qubits, in semiconductor quantum dots. The quantum dot charge qubit offers a simple architecture and high-speed operation, but generally suffers from fast dephasing due to strong coupling of the environment to the electron’s charge. On the other hand, quantum dot spin qubits have demonstrated long coherence times, but their manipulation is often slower than desired for important future applications. This talk will present experimental progress of a ‘hybrid’ qubit, formed by three electrons in a Si/SiGe double quantum dot, which combines desirable characteristics (speed and coherence) in the past found separately in qubits based on either charge or spin degrees of freedom. Using resonant microwaves, we first discuss qubit operations near the ‘sweet spot’ for charge qubit operation. Along with fast ( >GHz) manipulation rates for any rotation axis on the Bloch sphere, we implement two independent tomographic characterization schemes in the charge qubit regime: traditional quantum process tomography (QPT) and gate set tomography (GST). We also present resonant qubit operations of the hybrid qubit performed on the same device, DC pulsed gate operations of which were recently demonstrated. We demonstrate three-axis control and the implementation of dynamic decoupling pulse sequences. Performing QPT on the hybrid qubit, we show that AC gating yields π rotation process fidelities higher than 93% for X-axis and 96% for Z-axis rotations, which demonstrates efficient quantum control of semiconductor qubits using resonant microwaves. We discuss a path forward for achieving fidelities better than the threshold for quantum error correction using surface codes.
Hybrid semiconductor-superconductor qubits have recently emerged as a promising alternative to traditional platforms, combining material advantages with device-level tunability. A defining feature is their gate-tunable Josephson coupling, enabling superconducting qubit architectures with full electric-field control and offering a path toward scalable, low-crosstalk quantum processors. This approach seeks to merge benefits of superconducting and semiconductor qubits, for instance by encoding quantum information in the spin of a quasiparticle occupying an Andreev bound state, thus combining long coherence times with fast, flexible control. Progress has accelerated through bottom-up engineering of Andreev states in coupled quantum dot arrays, leading to architectures such as minimal Kitaev chains hosting Majorana zero modes. In parallel, Hamiltonian-protected designs aim to enhance resilience against local noise and decoherence by exploiting superconducting phase dynamics and discrete charge or flux degrees of freedom. This article reviews recent theoretical and experimental advances in hybrid qubits, providing an overview of physical mechanisms, device implementations, and emerging architectures, with emphasis on their potential for (topologically) protected quantum information processing. While many designs remain at proof-of-concept stage, rapid progress suggests practical demonstrations may soon be achievable.
Developing fast and accurate control and readout techniques is an important challenge in quantum information processing with semiconductor qubits. Here, we study the dynamics and the coherence properties of a GaAs/AlGaAs double quantum dot charge qubit strongly coupled to a frequency-tunable high-impedance resonator. We drive qubit transitions with synthesized microwave pulses and perform qubit readout through the state-dependent frequency shift imparted by the qubit on the dispersively coupled resonator. We perform Rabi oscillation, Ramsey fringe, energy relaxation, and Hahn-echo measurements and find significantly reduced decoherence rates down to γ_{2}/2π∼3 MHz corresponding to coherence times of up to T_{2}∼50 ns for charge states in gate-defined quantum dot qubits. We realize Rabi π pulses of width down to σ∼0.25 ns.
We discuss measurement and manipulation of a quantum dot hybrid qubit [1] formed in a Si/SiGe heterostructure. Xrotations on the Bloch sphere are performed by pulsing a gate voltage so that the detuning of a double quantum dot makes the (1,2) and (2,1) occupation ground states degenerate [2]. The resulting rotation rate is approximately 5 GHz and reveals an experimentally measured visibilty greater than 80 percent. Z-rotations on the Bloch sphere are performed by pulsing a gate voltage away from the (1,2)-(2,1) degeneracy point, resulting in oscillations at a rate of approximately 10 GHz and measured visibility greater than 85 percent. The T2* time at this detuning is greater than 15 ns, many times longer than the 100 ps gate operation time. In part because of the large ratio between the gate time and the dephasing time, improvements in the pulses used in the experiment are expected to enhance the visibility beyond that reported here and to enable high fidelity quantum gates. This work was supported in part by ARO (W911NF-12-0607), NSF (DMR-1206915), and the United States Department of Defense. The views and conclusions contained in this document are those of the authors and should not be interpreted as representing the official policies, either expressly or implied, of the US Government. This work was performed in collaboration with Dohun Kim, Zhan Shi, C. B. Simmons, D. R. Ward, J. R. Prance, Xian Wu, R. T. Mohr, Teck Seng Koh, John King Gamble, Ryan Foote, D. E. Savage, M. G. Lagally, Mark Friesen, and S. N. Coppersmith.
Abstract Identifying and ameliorating dominant sources of decoherence are important steps in understanding and improving quantum systems. Here, we show that the free induction decay time ( T2∗ T2* ) and the Rabi decay rate (ΓRabi) of the quantum dot hybrid qubit can be increased by more than an order of magnitude by appropriate tuning of the qubit parameters and operating points. By operating in the spin-like regime of this qubit, and choosing parameters that increase the qubit’s resilience to charge noise (which we show is presently the limiting noise source for this qubit), we achieve a Ramsey decay time T2∗ T2* of 177 ns and a Rabi decay time 1/ΓRabi exceeding 1 μs. We find that the slowest ΓRabi is limited by fluctuations in the Rabi frequency induced by charge noise and not by fluctuations in the qubit energy itself.